Liquid compositions, which hereinafter are often called "baths" for brevity, even though they may be used by spraying or other methods of establishing contact than immersion, that are useful for treating the surface of aluminiferous metals, defined as aluminum and its alloys that contain at least 50% by weight of aluminum, may be broadly classified into chromate-type treatment baths and non-chromate-type treatment baths. The chromate-type surface treatment baths typically are divided into chromic acid chromate conversion treatment baths and phosphoric acid chromate conversion treatment baths. Chromic acid chromate conversion treatment baths were first used in about 1950 and are still in wide use at present for the surface treatment of, for example, heat exchanger fins and the like. Chromic acid chromate conversion treatment baths contain chromic acid (i.e., CrO.sub.3) and hydrofluoric acid (HF) as their essential components and may also contain a conversion accelerator. These baths form a coating that contains small amounts of hexavalent chromium.
The phosphoric acid chromate conversion treatment bath was invented in 1945 (see U.S. Pat. No. 2,438,877). This conversion treatment bath contains chromic acid (CrO.sub.3), phosphoric acid (H.sub.3 PO.sub.4), and hydrofluoric acid (HF) as its essential components. The main component in the coating produced by this bath is hydrated chromium phosphate (CrPO.sub.4.4H.sub.2 O). Since this conversion coating does not contain hexavalent chromium, this bath is still in wide use at present as, for example, a paint undercoat treatment for the lid and body of beverage cans. However, these chromate type surface treatment solutions are environmentally problematic because the bath, unlike the coating formed with it, contains hexavalent chromium; therefore, the use of treatment solutions which do not contain hexavalent chromium is desirable.
The treatment bath taught in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho 52-131937 [131,937/1977] is typical of the non-chromate-type conversion treatment baths. This treatment bath is an acidic (pH=approximately 1.0 to 4.0) waterbome coating solution that contains phosphate, fluoride, and zirconium or titanium or their compounds. Treatment of aluminiferous metal surfaces with this non-chromate-type conversion treatment bath produces thereon a conversion film whose main component is zirconium and/or titanium oxide. The absence of hexavalent chromium is one advantage associated with the non-chromate-type conversion treatment baths; however, the conversion coatings produced by them in many instances exhibit a corrosion resistance and paint adherence that is inferior to those of the coatings generated by chromate-type conversion treatment baths.
The use of water-soluble resins in surface treatment baths and methods intended to provide aluminiferous metals with corrosion resistance and paint adherence is described, for example, in Japanese Patent Application Laid Open [Kokai or Unexamined] Numbers Sho 61-91369 [91,369/1986] and Hei 1-172406 [172,406/1989], Hei 1-177379 [177,379/1989], Hei 1-177,380 [177,380/1989], Hei 2-608 [608/1990], and Hei 2609 [609/1990]. In these examples of the prior art surface treatment baths and methods, the metal surface is treated with a solution containing a derivative of a polyhydric phenol compound. However, the formation of an acceptably stable resin-containing coating on the aluminiferous metal surface sometimes is highly problematic with these prior art methods, and they do not always provide an acceptable performance (corrosion resistance). The invention described in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 4-66671 [66,671/1992] constitutes an improvement to treatment methods that use polyhydric phenol derivatives, but even in this case the problem of an unsatisfactory adherence sometimes arises.
The surface of DI aluminum cans is at present treated mainly with the above-described phosphoric acid chromate surface treatment baths and zirconium-containing non-chromate surface treatment baths. The outside bottom surface of DI aluminum cans is generally not painted, but is subjected to high-temperature sterilization by immersion in boiling tap water. If the corrosion resistance of the aluminum is poor, it will become oxidized and darkened by components in the tap water. This phenomenon is generally known as "blackening".
Some aluminum DI cans are sterilized with high-pressure steam; however, a known problem of this process is whitening of the appearance by the growth of aluminum oxide crystals due to steam. In order to avoid this problem, the outer surface of the bottoms of aluminum DI cans sterilized with high-pressure steam has to be protected by painting. Ideally, the coating produced by surface treatment by itself, even when unpainted, would have to exhibit a high corrosion resistance.
Turning to another issue, a high friction coefficient for the can's exterior surface will cause the can surface to have a poor mobility during the conveyor transport that occurs in the can fabrication and finishing processes. This will cause the can to tip over, which will obstruct the transport process. Can transportability is a particular concern with regard to transport to the printer. Thus, there is demand in the can fabrication industry for a lowering of the static friction coefficient of the can's exterior surface, which, however, must be achieved without adversely affecting the adherence of the paint or ink which will be coated on the can. The invention disclosed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho 64-85292 [85,292/1989] is an example of a method directed to improving this mobility. This invention relates to a surface treatment agent for metal cans, wherein said surface treatment agent contains water-soluble organic substance selected from phosphate esters, alcohols, monovalent and polyvalent fatty acids, fatty acid derivatives, and mixtures of the preceding. While this method does serve to increase the mobility of aluminum cans, it affords no improvement in corrosion resistance or paint adherence. The invention described in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 5-239434 [239,434/1993] is another method directed to improving the mobility of aluminum cans. This invention is characterized by the use of phosphate esters. This method does yield an improved mobility, but again it affords no improvement in corrosion resistance or paint adherence.